High-temperature superconductor (HTS) coils can be used for pole windings of machine rotors, it being possible for them to be cooled from room temperature to operating temperature. The superconducting properties of a superconducting material contained in the coil winding in the HTS conductor develop at operating temperature. The rotor in this case is mounted on a shaft, with the high-temperature superconductor coil being cooled through, for example, a hollow shaft. The rotor of the rotating machine has several poles, the windings of which are cooled via the hollow shaft, with the result that the high-temperature superconductor (HTS) coils are cooled to the appropriate operating temperature.
Traditionally, mostly first-generation (1G HTS) high-temperature superconductors are currently used. Most such strip conductors are a few mm wide and only a fraction of a mm thick. In the case of 1G HTS, they contain filaments of a granular HTS ceramic (e.g. BiSrCaCuO) embedded in a silver matrix, this configuration being created using the so-called silver tube introduction method. With such conductors, no special steps need be taken to avoid compression.
Second generation (2G HTS) strip conductors now exist. These are manufactured in a coating process in keeping with coated conductor architecture. A thin film of the HTS ceramic material, e.g. YBCuO, is applied to a flexible, windable substrate material, for example a strip of an iron alloy (steel, e.g. Hastelloy) that is suitable for use at low temperature and highly flexible, or a nickel-tungsten alloy. In some cases, the production process for the 2G HTS strip also includes one or more intermediate coatings before the application of the superconductor film. Copper can also be applied to one or both sides of the conductor in order to stabilize it against excess currents.
Improvements compared to 1G HTS conductors, in particular higher current density, better mechanical properties and a greater choice of conductor materials and geometry, are expected of 2G HTS conductors.
When used with coils the HTS strip conductor (1G or 2G) is usually enclosed in electrical insulation and, in order to fix it mechanically, the whole is embedded (impregnation) in a resin. As the purpose of the HTS coils is to generate a magnetic field, Lorenz forces are then exerted on the individual HTS strip conductors.
When the high-temperature superconductor (HTS) coil cools down from room temperature to the operating temperature of the high-temperature superconductor (HTS) coil, thermal shrinkage of the coil winding occurs in a radial direction, but also in a circumferential direction. This thermal shrinkage is significantly greater than the free, uncontrolled shrinkage of the HTS conductor, as the impregnating resin (in most cases epoxy resin) used in the manufacture of the coils has a much higher shrinkage value, for example 1.4% of 300K to 30K, than the HTS conductor, which consists of substrate, HTS coating and copper and has a shrinkage value of 0.3% of 300K to 30K. This thermal shrinkage of the coil winding results in compression, in the longitudinal direction (circumferential direction) of the conductor, of the superconducting material contained in the coil winding. When coils made of 2G HTS material are manufactured using traditional methods, irreversible degradation of the superconducting properties of the superconducting material contained in the coil winding occurs as soon as the extent of longitudinal compression exceeds a specific value. The thin ceramic film in 2G HTS conductors is more susceptible to such compression than the filaments in 1G HTS conductors, which consist of many individual granules.